1. Field of the Invention
The present invention relates to an antiparallel (AP) pinned spin valve sensor with specular reflection of conduction electrons and, more particularly, to an AP pinned spin valve sensor where a specular reflecting layer reflects conduction electrons to a free layer of the sensor.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic fields from moving magnetic media, such as a magnetic disk or a magnetic tape. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer, and a free layer. First and second leads, connected to the spin valve sensor, conduct a sense current therethrough. The magnetization of the pinned layer is pinned 90.degree. to the magnetization of the free layer and the magnetization of the free layer is free to respond to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layers are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos .theta., where .theta. is the angle between the magnetizations of the pinned and free layers. A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor.
A read head employing a spin valve sensor (hereinafter referred to as a"spin valve read head") may be combined with an inductive write head to form a combined magnetic head. In a magnetic disk drive, an air bearing surface (ABS) of the combined magnetic head is supported adjacent a rotating disk to write information on or read information from the disk. Information is written to the rotating disk by magnetic fields which fringe across a gap between the first and second pole pieces of the write head. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
An improved spin valve sensor, which is referred to hereinafter as an antiparallel (AP) pinned spin valve sensor, is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein. The AP pinned spin valve sensor differs from a single pinned layer spin valve sensor, described above, in that the pinned layer of the AP pinned spin valve sensor comprises multiple thin films, which are collectively referred to as an antiparallel (AP) pinned layer. The AP pinned layer has a nonmagnetic spacer film sandwiched between first and second ferromagnetic layers wherein one of these layers may comprise several thin films. The first layer is exchange coupled to the antiferromagnetic layer (immediately adjacent thereto) and has its magnetic moment directed in a first direction. The second layer is immediately adjacent the free layer and, even though it is not immediately adjacent the first layer, it is exchange coupled thereto because of the minimal thickness (in the order of 8 .ANG.) of the spacer film therebetween. The magnetic moment of the second layer is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first layer.
The AP pinned layer is preferred over the single film pinned layer. The magnetic moments of the first and second layers of the AP pinned layer subtractively combine to provide a net pinning moment of the AP pinned layer. The direction of the net moment is determined by the thicker of the first and second layers. The thicknesses of the first and second layers are chosen to provide a low net magnetic moment. A reduced net moment equates to a reduced demagnetization (demag) field from the AP pinned layer. A reduced demag field also reduces the demag field imposed on the free layer which promotes a zero bias point for the operation of the free layer along its transfer curve. Further, since the antiferromagnetic exchange coupling is inversely proportional to the net pinning moment, exchange coupling between the first layer of the AP pinned layer and the pinning layer is increased by lowering the net pinning moment.
The high exchange coupling promotes higher thermal stability of the head. When the head encounters elevated thermal conditions caused by electrostatic discharge (ESD) from an object or person, or by contacting an asperity on a magnetic disk, the blocking temperature of the antiferromagnetic layer can be exceeded, resulting in disorientation of its magnetic spins wherein blocking temperature is the temperature at which the magnetic spins of the antiferromagnetic layer are free to rotate. The magnetic moment of the pinned layer is then no longer pinned in the desired direction. An increase in the exchange coupling decreases the instances of destabilization of the antiferromagnetic layer.
Efforts continue to increase the spin valve effect of GMR heads. An increase in the spin valve effect equates to higher bit density (bits/square inch of the rotating magnetic disk) read by the read head. Promoting read signal symmetry is a major consideration. This is accomplished by reducing the magnetic influences on the free layer. Another concern is reducing diffusion of conduction electrons in the AP pinned spin valve sensor without impacting other factors that are important to the performance of the sensor. It is known that one or more specular reflecting layers of nickel oxide (NiO), silver (Ag) or gold (Au) may be employed at the top and/or the bottom of a simple spin valve sensor for specular reflection of electrons so as to conserve conduction electrons in the spin valve sensor. A spin valve sensor is characterized as a top or bottom spin valve sensor depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (formed before the free layer). The aforementioned NiO is also a desirable material for a pinning layer in a bottom spin valve sensor. When a NiO layer is employed at the top of a simple spin valve sensor for specular reflection of electrons the prior art teaches interfacing the NiO layer with a copper layer so that the NiO layer does not exchange couple with the free layer.